a. Field of the Invention
Broadly speaking, this invention relates to waveguides. More particularly, in a preferred embodiment, this invention relates to multimode optical waveguides wherein intermodal coupling is deliberately enhanced to reduce dispersion in the guide.
B. Discussion of the Prior Art
The desirability of deliberately enhancing coupling between the various modes propagating in a multimode optical waveguide has been widely discussed in the literature. See, for example, the article entitled "Reduction of Multimode Pulse Dispersion by Intentional Mode Coupling" by D. Marcuse, The Bell System Technical Journal, Vol. 53, No. 9, (November 1974), pp. 1795-1815, U.S. Pat. No. 3,909,110 which issued to D. Marcuse on Sept. 30, 1975, U.S. Pat. No. 3,666,348 which issued to E. A. J. Marcatili on May 30, 1972 and U.S. Pat. No. 3,687,514 which issued to S. E. Miller et al. on Aug. 29, 1972, all of which commonly-assigned patents and the cited BSTJ article are hereby incorporated by reference.
As taught in the above-identified patents and in the literature, to reduce dispersion in a waveguide longitudinal perturbations may be induced into the guide, the spatial periodicities of which correspond, for example, to the beat frequency wavelengths between the different modes propagating in the guide. The perturbations which are induced into the guide may be continuous or discrete and may comprise physical changes in the cross-section of the guide and/or periodic changes in the index of the core or cladding. Marcuse, in the cited article and in U.S. Pat. No. 3,909,110, has shown that, to avoid radiation losses, conditions should be imposed not only on the axial dependence of the fluctuations, but also on the azimuthal and radial dependence of the fluctuations.
My invention, in a preferred embodiment, comprises a waveguide of the type which is capable of guiding wave energy in a plurality of different modes of wave propagation having differing group velocities. The waveguide according to my invention is characterized by means for enhancing the coupling between guided modes, the enhancing means having a sharp cutoff near f.sub.2, the critical spatial frequency at which energy is coupled to unguided modes.
This invention relates specifically to ways to realize the appropriate axial fluctuations and therefore the imposition of appropriate conditions on azimuthal and/or radial dependence of the fluctuations is assumed and is not explicitly considered herein.
The invention and its mode of operation will be more fully understood from the following detailed description, when taken with the appended drawings, in which: